The invention pertains generally to an excavating tool such as, for example, a rotary drill bit useful for drilling through various earth strata. More specifically, the invention pertains to a rotary drill bit with a cutting insert such as, for example, a roof drill bit useful for drilling bore holes in an underground mine.
The expansion of an underground mine, such as for example, a coal mine, requires digging a tunnel. Initially, this tunnel has an unsupported roof. Because the roof is not supported, there is an increased chance for a mine cave that, of course, adds to the hazards of underground coal mining. Furthermore, an unsupported roof is susceptible to rock and debris falling from the roof. Falling rock and debris can injure workers as well as create hazardous clutter on the floor of the tunnel. In order to support and stabilize the roof in an underground tunnel, bore holes are drilled in the roof, i.e., earth strata.
The apparatus used to drill these holes typically comprises a drill with a long shaft, i.e., drill steel, attached to the drill. A roof drill bit is detachably mounted to the drill steel at the distal end thereof. In certain roof drill bits, a hard cutting insert is mounted on a body of the roof drill bit. The roof drill bit is then pressed against the roof, and the drilling apparatus operated so as to drill a bore hole in the roof. The bore holes extend between about two feet and about twenty feet into the roof depending upon the particular situation. The roof support members, such as roof panels, are then attached to roof bolts. In one alternative procedure, these bore holes are filled with resin and roof bolts are fixed within the bore holes. In another alternative procedure, the roof bolts use mechanical expander shells to affix the roof bolts in the bore holes. The end result of using either procedure is a roof which is supported, and hence, is of much greater stability than the unsupported roof. This reduces the hazards associated with underground mining. The roof bolting process is considered to be an essential underground mining activity.
Roof bolting accounts for the largest number of lost time injuries in underground mining. During the roof bolting process, the roof is unsupported so that it does not have optimum stability. Furthermore, the roof bolting process exerts stresses on the roof so as to further increase the safety hazards during the roof bolting process. Thus, a decrease in the overall time necessary to bore holes reduces the time it takes to complete the roof bolting process. This is desirable since it contributes to the overall speed, efficiency and safety of the roof bolting process. Thus, many solutions have been proposed to decrease the overall time to complete the drilling of the necessary bore holes. For example, roof drilling bits with various cutting inserts and various cutting geometries have been developed. Efforts have also been made to increase the overall useful life of roof drilling bits.
FIGS. 1 and 1A set forth an example of a known cutting insert 2 (for use with a roof drilling bit) having a leading face 4, a top surface 6 that includes a primary relief surface 7 adjacent to the leading face 4 and secondary relief surface 8. The leading face 4 and the primary relief surface 7 intersect at an angle M (90 degrees or less) to form a cutting edge 9 which results in a positive or neutral axial rake angle N. However, it has been determined that this configuration results in a large amount of stress on the cutting edge 9 which in turn leads to the failure of the cutting edge 9 (e.g. as a result of breaking or chipping of the cutting edge) and, thus, the cutting insert 2 needing to be replaced.
Accordingly, there is a need for improved roof drilling bits that overcome disadvantages, limitations and shortcomings of known roof drilling bits. For example, it would be desirable to provide an improved roof drill bit that facilitates the prompt completion of the roof bolting process. It would also be desirable to provide an improved roof drill bit that has a longer useful life. It would also be desirable to provide an improved roof drill bit that has an increased penetration rate.